1. Field of the Invention
This invention relates to charge transfer devices and more particularly is directed to a charge transfer device employing a CCD (charge coupled device).
2. Description of the Prior Art
When a CCD is employed as a solid state image pickup element or sensor, it is natural that high resolution should be expected. Considering that a CCD is incorporated into a camera, its chip size has to be made small in view of a manufacturing cost of an optical system. In order to reduce the chip size and increase the number of picture elements, it is necessary first to reduce the area of one picture element in the light receiving region of the CCD. When the chip size and the number of picture elements in the horizontal direction are determined, the transverse width of one picture element is determined. In accordance with the present manufacturing technique, the transverse width of one picture element can be formed in several microns. However, in the case of the CCD image pickup device of, for example, frame transfer system, the pitch per one bit of a horizontal register which will read out an information signal accumulated in the light receiving region has to be formed by the transverse width of one picture element, giving rise to difficulty in the manufacturing technique. Moreover, when the CCD is employed as a memory, particularly SPS (serial-parallel-serial) type memory, the serial section has to be manufactured following the width of the parallel section so that the same problem occurs.
In order to improve the aforesaid defects, the assignee who is the same as that of this invention has previously proposed a charge transfer device of zigzag-channel type employing the CCD in which the length per one bit of the horizontal register can be reduced.
FIG. 1 schematically illustrates such previously proposed charge transfer device of zigzag-channel type. As shown in FIG. 1, the charge transfer device of zigzag-channel type comprises storage electrodes 1 and 2, transfer regions 3 and 4 and storage regions 5 and 6. In this case, the storage regions 5 and 6, the depth of the potential well of which is determined by the storage electrodes 1 and 2 are opposed to each other and therebetween are located the transfer regions 3 and 4 of which the depth of the potential well is determined by transfer electrodes (not shown). The storage regions 5 and 6 are respectively encircled on three sides by channel stopper regions 7 and a signal charge is transferred in a zigzag way from the storage region 5 - the transfer region 4 - the storage region 6 - the transfer region 3 . . . as, for example, shown by a broken line arrow 8. With this arrangement, the ends of the opposing storage electrodes 1 and 2 facing to the transfer regions 3 and 4 form straight lines a-a' and b-b', respectively. The lengths in which the storage regions 5 and 6 contact with the transfer regions 3 and 4 are presented as c-e and h-j, respectively. When the charge is transferred from the storage region 5 to the storage region 6, the length of exit through which the charge is sent becomes a short line c-d and the length of entrance through which the charge enters becomes a short line h-i. For this reason, two-dimensional effect by the potential in the transfer region 4 on the potential in the storage region 5 is poor and a fringing electric field is difficult to be established in the direction of the transfer region 4. Similarly, a fringing electric field from the transfer region 4 to the storage region 6 is difficult to be established. A minimum distance k-l between the opposing portions of the channel stopper regions 7 is shorter than the minimum distance between the storage regions 5 and 6 and a potential barrier due to the potential in the channel stopper regions 7 easily appears on the line k-l in the transfer regions 4. Thus, there occurs such a case that the charge can not be transferred completely sometimes and the charge transfer efficiency is deteriorated. Since the shapes of the channel stopper regions 7 encircling the storage regions 5 and 6 are symmetrical as c-e-f-g and the potentials in the channel stopper regions 7 do not work positively for transferring the charge in the storage regions 5 to the c-d direction rather than d-e direction, the charge transfer efficiency is deteriorated.
With this prior art arrangement, when an error occurs in aligment of the masks and as shown in FIG. 2, the transfer regions 3 and 4 are shifted to left-hand side relative to the channel stopper regions 7, the length of exit of the storage region 5 to the transfer region 4 becomes narrower as in c'-d'. Accordingly, the aforesaid tendency becomes serious and the fringing electric field is made more difficult to be established so that the charge transfer efficiency is deteriorated.